prefer doxygen @copydoc over duplicating documentation

[tjhei]

We have many duplicated lines of documentation, especially in the material models, that doesn't make a lot of sense. Example: simple.h is_compressible().

[pmbremner]

@tjhei Does the nature of this issue change now that everything is switching over to Sphinx?

[bangerth]

No, this is still the case. We are only changing from latex to sphinx for the hand-written manual, but the automatically generated doxygen documentation remains on that system. So what @tjhei points out above remains true.